The present invention is related to disc drives. In particular, the present invention is related to servo systems.
In a computer disc drive, data is stored on a computer disc in concentric tracks. In disc drives with relatively high track densities, a servo feedback loop is used to maintain a head over the desired track during read or write operations. This is accomplished utilizing prerecorded servo information either on a dedicated servo disc or on sectors that are interspersed along a disc. During track following, the servo information sensed by the head is demodulated to generate a position error signal (PES), which provides an indication of the distance between the head and the track center. The PES is then converted into an actuator control signal, which is used to control an actuator that positions the head.
Historically, only one actuator, typically a voice coil motor, was used to position the head. Recently, micro-actuators have been proposed that would be used in combination with the voice coil motor to position the head. Because they are small, such micro-actuators generally have a better frequency response than the voice coil motor. As such, they are better able to follow high frequency control signals.
Although the micro-actuators have a better frequency response than voice coil motors, they also have a more limited range of motion. When a micro-actuator reaches the limit of its range of motion, increases in the control signal to the micro-actuator do not result in any more movement of the micro-actuator. Under these conditions, the micro-actuator is said to be saturated.
When a micro-actuator saturates in a servo loop, it causes two problems. First, because the stability of the servo loop is designed based on the inclusion of a responsive micro-actuator, when the micro-actuator saturates, it can cause the entire servo loop to become unstable. Second, the actuator controller corresponding to the micro-actuator typically includes some form of past state feedback. When the micro-actuator is unable to move to the position set by the controller, the past state stored in the controller is no longer tied to the movement of the head. This discrepancy in the past states affects the performance of the controller even after the input to the controller drops below a value that would cause the micro-actuator to: saturate. The errors caused by past state discrepancies are known as windup.
Prior art systems have attempted to avoid instability and windup by modifying the gains of the micro-actuator and the voice-coil motor controllers. Although such systems may prevent micro-actuator saturation, they also decrease the overall performance of the servo loop because they make the micro-actuator less responsive to control signals that would not saturate the micro-actuator.
Thus, a servo system is needed that prevents the instability and wind-up associated with micro-actuator saturation but does not affect the performance of the servo loop when the micro-actuator is not saturated.
A method and apparatus are provided for positioning a head over a disc in a disc drive while maintaining servo loop stability. The apparatus includes an actuator-head assembly having a large scale actuator and a micro-actuator that are both able to move the head over the disc. An anti-windup compensation component detects when a micro-actuator controller is producing a micro-actuator control value that will saturate the micro-actuator. Using the micro-actuator control value, the anti-windup compensation component generates a saturation tracking error signal. A summing component then combines a position value that the head generates based on its position over the disc with the saturation tracking error signal to form a position error value that masks the saturation of the micro-actuator.